Flux cored wire for gas shielded arc welding

ABSTRACT

Disclosed is a flux cored wire for gas shielded arc welding, which is capable of offering excellent workability and providing a weld metal with an improved impact value at low temperature. The flux cored wire for gas shielded arc welding is characterized in that a titania-based flux composition is filled into a sheath for mild steels of low-carbon steels, and the flux composition contains 4.0 to 8.0 % TiO 2 , 1.0 to 3.0 % Mn, 0.1 to 1.0 % Si, 0.002 to 0.02 % B, 0.2 to 0.45 % Mg and 0.15 to 0.3 % Al, by weight relative to the total weight of the wire.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a flux cored wire, and moreparticularly to a flux cored wire for gas shielded arc welding, which isfilled with a titania-based flux composition and employs carbon dioxideor a mixture of carbon dioxide and argon as a shield gas.

[0003] 2. Description of the Related Art

[0004] In general, titania-based flux cored wires have been mainlyapplied to fabricate mild steels and high tensile strength steels havinga tensile strength of 50 kgf/mm² or more. Although the titania-basedflux cored wires provide very excellent workability, there is adisadvantage in that upon low-temperature welding or high heat inputwelding, such conventional wires provide weld metals with poor impacttoughness.

[0005] Indeed, the application of such titania-based flux cored wireshas problems in ship fabrications, since the grade III of ship steelstandards is strictly specified with respect to the impact values ofmetals(that is, more than 55 J at −20), and in steels for a lowtemperature use, such as aluminum killed steels (that is, more than 28 Jat −40 to −50).

[0006] Meanwhile, using cored wires filled with a basic flux, suchproblems could be overcome. However, although weld metals have asatisfactory impact value at low temperature, some problems stillremain, including that welding in all positions is unstable in terms ofworkability, and welding spatter and fume generation are increased.

SUMMARY OF THE INVENTION

[0007] Therefore, the present invention has been made in view of theabove problems, and it is an object of the present invention to providea flux cored wire for gas shielded arc welding, which is capable ofoffering excellent workability and providing a weld metal with animproved impact value at low temperature.

[0008] In accordance with the present invention, the above and otherobjects can be accomplished by the provision of a flux cored wire forgas shielded arc welding, characterized in that a titania-based fluxcomposition is filled into a sheath for mild steels of low-carbonsteels, and the flux composition contains 4.0 to 8.0% TiO₂, 1.0 to 3.0%Mn, 0.1 to 1.0% Si, 0.002 to 0.02% B, 0.2 to 0.45% Mg and 0.15 to 0.3%Al, by weight relative to the total weight 100 % of the wire.

[0009] More preferably, the content ratio of Mg to Al may be 1.45 to1.55.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0011]FIG. 1 is a perspective view showing a test plate for welding in avertical upward position to test a welding workability, with respect toexamples of the invention and comparative examples;

[0012]FIG. 2a is a plan schematic of a test plate used for testing hightemperature cracks in a welded portion, with respect to examples of theinvention and comparative examples; and

[0013]FIG. 2b is a sectional schematic as viewed from the plane of theline A-A in FIG. 2a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In accordance with the invention, a flux composition is filledinside a sheath for mild steels and is compounded of arc stabilizers,slag forming agents, deoxidizing agents and alloy additives, withrespective functions thereof.

[0015] Under the consideration of a covering property and fluidity ofslag, as an oxide, the TiO₂ component should be contained in the fluxcomposition in an amount of 4.0 to 8.0% by weight, relative to the totalweight of the wire.

[0016] As for the deoxidizing agents, Fe-Mn, Me-Mn and other Mn alloyirons are added to the flux composition, such that the total Mn amountis in a range of 1.0 to 3.0% by weight, relative to the total weight ofthe wire.

[0017] Taking account of a deoxidizing power and viscosity of slag,Fe-Si, Fe-Si-B and the like are incorporated, such that the total Siamount is in a range of 0.1 to 1.0% by weight, relative to the totalweight of the wire. On the other hand, B should be adjusted to a weightof 0.002 to 0.02%, relative to the total weight of the wire. With such aproportion of the components in the flux composition, the coveringproperty and fluidity of slag remain constant, thereby making possiblestable welding in all positions, and further improving an impact valueat low temperature.

[0018] In addition, with an aim of improving an arc stability and beadshape, Mg and Al are incorporated. The content ratio of Mg to Al (i.e.,Mg/Al) should meet Equation 1 below, thus improving the arc stabilityand improving power of concentration of the arc. Further, Mg and Al areeffective to further improve the above-described properties of the aboveflux composition, whereby the welding in all positions can be performedin a convenient manner. Also, they can provide the weld metal with muchmore improved impact toughness at low temperature.

[0019] [Equation 1]

1.45≦Mg/Al≦1.55

[0020] Sources of Mg and Al include Mg-Al, Me-Mg, Fe-Al, Me-Al and thelike.

[0021] Next, with regard to the flux composition of the invention, thereasons that the contents of its components are numerically limited willbe described in detail.

[0022] A main component of the titania-based flux composition, TiO₂, isof high importance, acting as a slag forming agent and an arcstabilizer. It gives both an excellent covering property anddetachability of the slag, even though such properties are uncommon toother slag forming agents.

[0023] However, with TiO₂ at less than 4.0% by weight relative to thetotal weight of the wire, it fails to obtain good appearance and shapeof a bead. Also, poor slag coverage is obtained, and the arc stabilityis deteriorated, while the amount of spatter generation increases. Onthe other hand, if the content of TiO₂ exceeds 8.0% by weight, the slaggeneration is excessive, thickening the slag layer. Moreover, TiO₂ canremain as a non-metal inclusion, upon slag floating over a molten pool,or it increases a risk of slag inclusion in the weld metal.

[0024] As a source material for TiO₂, rutile sand, reduced ilmenite,titanium oxide or leucoxene may be used.

[0025] Mn is incorporated as a deoxidizing agent, and also to play anadditional role of reducing the oxygen content in the weld metal,thereby improving low temperature toughness.

[0026] However, if the content of Mn is less than 1.0% of the totalweight of the wire, the deoxidizing ability is not sufficient, so thearc is made unstable, or the blowholes are easily formed. On the otherhand, if Mn is over 3.0%, the strength of the weld metal is outside ofthe specified standard for the application, or high temperature crackseasily occur.

[0027] Therefore, it is preferred that the content of Mn is set to 1.0to 3.0%, relative to the total weight of the wire. The source materialmay be Fe-Mn, Me-Mn, Si-Mn and the like.

[0028] Si is also effective as a deoxidizing agent so as to reduce theoxygen content in the weld metal. Si is oxidized to SiO₂ slag with goodfluidity and a covering property. The slag viscosity can be adjusted soas to make easier welding in a vertical upward position.

[0029] However, if the content of Si is less than 0.1% relative to thetotal weight of the wire, the deoxidizing ability is not sufficient,causing welding defects such as generation of blowholes. Due to a poorcoverage and viscosity of the slag, the applicability to a welding workin a vertical upward position is lowered. On the other hand, if it isover 1.0%, the content of Si in the weld metal increases, promotingcreation of a sigma phase, thereby deteriorating low temperaturetoughness. Accordingly, care should be taken to prevent suchdysfunctional effects of Si from being exerted. Further, in a course ofwelding, the amounts of spatter and welding fume generation can beincreased. For this reason, it is preferred that the Si content islimited to 1.0% or less. The source material may be Fe-Si, Fe-Si-Mn orthe like.

[0030] B is an element that makes the structure of a weld metal fine,having an effect of improving low temperature toughness. The amountemployed should be specially noted, since if the B content is over alimited range, the weld metal is rapidly hardende, causing hightemperature cracking.

[0031] Thus, the limiting range of the B amount is preferably 0.002 to0.02% by weight, relative to the total weight of the wire.

[0032] Mg serves as an arc stabilizer and a strong deoxidizing agent. Itis converted into MgO inside the arc, via its rapid oxidation. MgOincreases a solidifying point of the slag, making possible a good weldappearance without a rough bead surface in a vertical upward weldingposition.

[0033] However, if the content of Mg is less than 0.2% relative to thetotal weight of the wire, its effects are little exerted. On the otherhand, if it is over 0.45%, the amount of spatter generation isincreased, and slag formation is weakened, thereby worsening the beadshape. Thus, the Mg content should be limited to 0.2 to 0.45 w %,relative to the total weight of the wire.

[0034] Al also plays a role as a deoxidizing agent. With a combinationof functions of Fe-Mn and Fe-Si, it helps slag formation and preventsthe molten metal from flowing away. However, if the Al content is over0.3 w %, relative to the total weight of the wire, the bead shape isworsened. Such an Al content adversely affects arc formation and lowerslow temperature toughness of the weld metal. Thus, it is preferred thatthe Al content should be 0.15 to 0.3 w %, relative to the total weightof the wire.

[0035] In accordance with the invention, to improve weldability in allwelding positions and make a vertical upward position more convenientfor welding, the mixing ratio of Mg and Al should be more than 1.45 andless than 1.55. By employing such a ratio, welding efficiency due to arcstability can be improved, and it is possible to obtain a metal with alow temperature toughness of more than 50 J which is applicable toaluminum killed steels at −40 With an adequate combination of Mg and Al,the functions of other deoxidizing agents can be boosted. Moreover, sucha combination allows an adjustment of a melting point of slag, therebybeing excellent in its applicability in all welding positions. However,if the ratio of Mg and Al departs from the range of the invention,though ratios of other components meet the compositions for a flux ofthe invention, the beneficial effects of Mg and Al are likely to besignificantly decreased.

[0036] Hereinafter, the examples according to the present invention willbe described in detail, as compared with comparative examples.

[0037] Specimens were prepared, provided that the examples are wireswhose components of the flux composition and the proportions thereof arein the limiting range according to the invention. For comparativeexamples, the specimens are made, provided that at least one componentis contained in the flux in a proportion outside the scope of thepresent invention.

[0038] A variety of flux compositions, relative to examples orcomparative examples, are represented in Table 1.

[0039] Specimens having different flux compositions with a filling rateof 15% in the sheath, and having a diameter of 1.2 mm were prepared andevaluated.

[0040] Using the wires thus prepared, test plates were subjected towelding under the welding conditions represented in Table 2. Tests wereperformed to assess arc stability amounts of spatter and fume generationand slag detachability. TABLE 1 Flux compositions Chemical compositions(w %) Specimen TiO₂ Mn Si Al Mg B Mg/Al Ex. 1 6 2.5 0.3 0.2 0.3 0.0051.5 2 6 2 0.3 0.2 0.3 0.005 1.5 3 6 2.5 0.4 0.15 0.22 0.005 1.47 4 6.52.5 0.4 0.15 0.22 0.005 1.47 5 6.5 2 0.55 0.15 0.23 0.005 1.53 6 6.5 20.55 0.18 0.27 0.005 1.5 7 7 2 0.55 0.18 0.27 0.005 1.5 8 7 2 0.6 0.220.34 0.005 1.55 Comp. 9 6 0.5 0.3 0.25 0.3 0.005 1.2 Ex. 10 8.4 1.5 0.30.2 0.3 0.005 1.5 11 3.6 2.5 0.4 0.2 0.22 0.005 1.1 12 6.5 2 1.4 0.10.23 0.005 2.3 13 6.5 2 0.3 0.1 0.3 0.005 3 14 4 2.5 0.4 0.15 0.3 0.0052 15 7 2.5 0.5 0.2 0.34 0.03 1.7 16 7 2 0.5 0.2 0.3 0.03 1.5

[0041] TABLE 2 Welding condition Parameters Welding conditions Testplate material Rolled steels for welding structures SM490A Test platematerial thickness 12, width 100, length 300 dimensions Welding positionFlat and horizontal fillet Vertical upward welding Welding current (A)260-340 200-240 Welding voltage (V) 28-35 24-25 Welding speed 40 15-20 (/min) Shield gas 100% CO₂ Shield gas flow rate 20 l/min

[0042] Further, in a vertical upward welding position, a T-shaped testplate illustrated in FIG. 1 was subjected to a continuous build-up of abead (in FIG. 1, the numerals 1 and 2 denotes a base metal, a weld bead,respectively, and D denotes a welding direction). Several parametersincluding a flowing amount of a weld metal in welding and drooping ofthe bead were observed with the naked eye and evaluated. The results areshown in Table 3. TABLE 3 Welding results Welding workability Horizontalfillet position Vertical upward position Arc spatter fume Bead Arcspatter fume Bead Specimen stability generation generation Shapestability generation generation Shape Ex. 1 2 3 O O 4 O O O O O O 5 O OO O 6 O O 7 O O O 8 O O O O O Comp. 9 O O O O X Ex. 10 O O O O O 11 O OO X O O O X 12 X X X 13 O O X O O X 14 O O O O O X 15 O O O O O * 18 O OO O O O *O 

[0043] To evaluate properties of weld metals, test plates weremanufactured according to the procedures of American welding Society(AWS) standard, under the welding conditions shown in Table 4. Themechanical and physical tests of the weld metals were performed.

[0044] For testing high temperature cracks in the weld metals, the testplates as illustrated in FIGS. 2a and 2 b, which is a kind of steelsimilar to the plates for mechanical and physical tests, weremanufactured. Giving 6 mm of a root gap, welding was performed at awelding speed of 24 cm/min using 250 A/31 V. After slag was removed by aliquid penestration technique, it was determined whether the bead cracksappear or not (in FIGS. 2a and 2 b, the numerals 3, 4 and 5 denote testplate, weld bead, a restraining base and a backing of ceramic material,respectively, and D denotes a welding direction).

[0045] The results of testing tensile strength, cold impact and hightemperature cracks are shown in Table 5. TABLE 4 Welding conditionsParameter Welding condition Test plate steels Rolled steels for weldingstructures SM490A Test plate dimensions Thickness 19, width 150, length300 Groove angle 45° Root space  12 Numbers of pass and layer 17 passes6 layers The temperature between layers 150 Shield gas 100% CO₂ or 80%Ar + 20% CO₂ Welding current 240 to 260 A Welding voltage 26 to 30 V

[0046] TABLE 5 Properties of weld metal Tensile Impact value Cracks athigh Specimen strength( f/ ) at −40 (J) temperature Ex. 1 580 53.8 No 2590 54 No 3 590 56 No 4 605 54 No 5 605 53.6 No 6 620 55 No 7 620 55 No8 620 54 No Comp. 9 540 24 No Ex. 10 560 23 Yes 11 590 48 Yes 12 680 21Yes 13 610 48 No 14 600 42 No 15 620 56 Yes 16 620 54 Yes

[0047] As can be seen from Table 1, the wires of examples 1 to 8, inwhich the proportions of the chemical components for respective fluxcompositions were within a limiting range according to the presentinvention. They exhibited good welding workability and produced verygood bead appearance without drooping or flowing of the weld metals evenin a vertical upward position.

[0048] As revealed in tests analyzing properties of the weld metals, thefinished metals had a satisfactory tensile strength, and also as forcold impact values, the metals meet the specified AWS standards at −40.

[0049] As for the test for high temperature cracks, good results wereobtained in which the weld metals did not generate cracks in their weldbead.

[0050] On the other hand, with regard to the comparative wires in whichat least one component in the flux composition is outside the scope ofthe present invention in terms of its composition and proportion,welding workability was poor. In addition, satisfactory impact values ofthe weld metals, which are evaluated as one of mechanical and physicalproperties, were not reached.

[0051] In particular, as for the wires of comparative example 9, as adeoxidizing agent, Mn was deficient, and the ratio of Mg/Al was out ofthe range defined in the present invention. The welding employing such awire increased the occurrence of blowholes due to insufficientdeoxidization, and lowered arc stability, and reduced welding efficiencybecause of the occurrence of bead droops in a vertical upward position.As for the wires of comparative example 10, welding workability wassubstantially good, but cold impact property fails to meet therequirements. The shape of bead, also, was somewhat rough.

[0052] As for the wire of comparative example 11, the amount of TiO₂ andthe ratio of Mg/Al were out of the range defined in the presentinvention. Slag coverage was bad due to deficiency of the amount ofslag, and bead droops occurred in a vertical upward position due toinsufficient viscosity of the melt slag.

[0053] Finally, as for the wires of comparative examples 12 to 15, theratio of Mg/Al were out of the range defined in the present invention.The amount of fume generation was increased, and the bead was driventoward one corner, adversely affecting the weldability. Especially, asfor the examples 15 and 16, the occurrence of cracks in the weld bead athigh temperature was increased due to the excess amount of B added.

[0054] As described in the above, the flux cored wire for gas shieldedarc welding according to the invention offers excellent workability inall welding positions and can provide a weld metal with good lowtemperature toughness.

[0055] Further, such a flux cored wire has effects of making weldingefficiency excellent, providing good shape of the bead, arc stability,and minimizing the amount of spatter generation.

[0056] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A flux cored wire for gas shielded arc welding,characterized in that a titania-based flux composition is filled into asheath for mild steels of low-carbon steels, and the flux compositioncontains 4.0 to 8.0% TiO₂, 1.0 to 3.0% Mn, 0.1 to 1.0% Si, 0.002 to0.02% B, 0.2 to 0.45% Mg and 0.15 to 0.3% Al, by weight relative to thetotal weight of the wire.
 2. The flux cored wire as set forth in claim1, which is characterized in that a content ratio of Mg to Al is 1.45 to1.55.